The document DE-A1-3,544,609 proposes a shutoff device, the gasket of which is mounted on an annular support axially displaceable towards the closing element under the action of pneumatic fluid. In a shutoff device of this type, when the closing member is being actuated the friction of the latter on the gasket must be avoided as far as possible, in order to prevent a rapid wear of the gasket. The gasket support is therefore designed in the form of an annular pneumatic jack which is delimited radially by an internal concertina compensator and an external concertina compensator. The gasket is laid onto the closing element by putting the pneumatic jack under pressure, the displacement of the gasket being allowed by the expansion of the corrugations of the two compensators. For displacement in the opposite direction the pneumatic jack is ventilated, and the natural elasticity of the corrugations of the compensator is utilized in order to lift the gasket from the closing element. Now because spring functions are assigned to the compensator, this not being its primary purpose, it is necessary to take into account that, in contrast to a spring, a compensator cannot be prestressed, and therefore a certain number of corrugations is required to ensure the desired displacement, thereby increasing the height of the annular pneumatic jack.
These devices, which are used particularly in systems for the pneumatic transport of, for example, coal dust, must be capable of containing the pressure both upstream and downstream of the closing element. In the known device, the surface of the gasket support which is exposed to the pressure in the housing corresponds substantially to the annular surface undergoing the action of the pneumatic fluid, particularly where the wear of the gaskets is concerned. To make it possible to ensure effective closing against the pressure inside the housing, the pressure of the pneumatic fluid must be substantially higher than the pressure of the pneumatic transport system, the more so because the pulling force of the compensator, occurring in the opposite direction, has to be overcome. In other words, the known device does not make it possible to use the main pressurizing station of the transport system in order to actuate the pneumatic jack, this being because a higher pressure is needed. It is therefore necessary to have separate compressor in order to actuate the pneumatic jack.